1. Field of the Invention
The present invention relates to an electric power steering apparatus so constructed as to perform steering assist by transmitting a driving force produced by an electric motor to a steering mechanism.
2. Description of Related Art
Conventionally, electric power steering apparatuses that perform steering assist by transmitting torques produced by electric motors to steering mechanisms in vehicles have been employed. The electric motors are driven and controlled on the basis of target currents determined depending on steering torques applied to steering wheels and vehicle speeds.
However, the efficiencies of the steering mechanisms change depending on temperatures and required axial forces (forces in the axial direction which should be produced by rack shafts of the steering mechanisms). Notwithstanding, the changes in the efficiencies are not taken into consideration in the conventional electric power steering apparatuses. Consequently, steering assist forces may, in some cases, be excessive or deficient depending on the ambient temperatures of the steering mechanisms and the magnitudes of loads. Accordingly, good steering feelings may not be obtained.